This invention relates to fluid directing elements for turbines, in particular to nozzles for high temperature gas turbine engines.
In each turbine stage of a gas turbine engine, a nozzle directs and accelerates hot pressurized combustion gas into a bladed wheel to perform work upon the blades. Nozzles comprise an annular array of fluid directing, airfoil shaped vanes fixed at their ends to inner and outer shrouds. since the first stage turbine nozzle directs the gas immediately emerging from the combustor it is exposed to the most extreme temperatures and requires highly sophisticated materials and thermal and structural design. In high temperature engines, the nozzles in the first one or two turbine stages are not only cast from the most temperature resistant superalloys available, but are also cooled by air flowing in passages within the vanes and by air dispersed over the exterior surfaces of the vanes and the shrouds. The nose and tail of a vane airfoil are most intensely heated by the flowing hot gas and consequently are designed to receive the strongest cooling.
Cooling of a vane nose is often accomplished by casting a hollow vane and positioning within the hollow an insert from which compressed air impinges upon the interior of the vane nose. The spent impingement air is ducted within the hollow around the insert and discharged through holes in the vane walls and vane tail to film cool the exterior surfaces of the vane. The assembly and sealing of the insert into the vane hollow and the drilling of the film cooling holes are costly operations.
A conventional method of nozzle fabrication is to cast the vanes separately and then braze, weld or mechanically fasten them to the inner and outer shrouds. Brazing, the cheapest attachment method, is structurally weak. Conventional welding is not much stronger, and electron beam welding is very costly. Mechanical retention of the vanes in the shrouds is also costly and produces a heavy design.
Integral casting of the vanes and shrouds is a commonly used method of fabrication which solves many of the vane attachment problems but limits casting process flexibility and cooling configuration complexity. Also, in an integral casting, some locations on the vanes where cooling holes are desirable are inaccessible to drilling owing to interference from adjacent vanes.
The significant increase in gas turbine efficiency and power obtainable with increasing turbine inlet temperature has spurred considerable effort over the years to develop turbines capable of accepting higher temperatures. Most of the effort has been directed toward improving the high temperature properties and cooling of metallic turbine hardware such as nozzles. New processes have been developed recently which, through unidirectional solidification of superalloy castings, produce columnar-grained material with high temperature properties improved over conventionally cast, equiaxed-grain material. A process has also been demonstrated for the casting of monocrystalline turbine elements which are still more superior in high temperature properties. However, the casting of integral nozzles using these new and costly processes is not economically practical because the intricacy of an integral nozzle makes it inherently subject to many casting defects and high scrappage rates.
Recently, interest has heightened in employing ceramics in gas turbines. The high temperature capabilities of ceramic materials are very attractive, but the brittleness, the poor tensile strength, and the problems of mating these materials to metal have prevented their use to date.
An object of my invention is a gas turbine nozzle with longer life, higher temperature capability and reasonable fabrication cost.
Another object is a gas turbine nozzle configured so that materials with superior high temperature properties can be practically and economically utilized, particularly in those areas where current nozzle configurations experience impairment from the high temperature fluids they direct.
Another object is a nozzle which can be readily and inexpensively provided in desirable locations with holes and passages for cooling air.
Still another object is a nozzle in which those portions most intensely heated and subject to damage are replaceable to restore the utility of the nozzle.